Radiotelephone communication using mobile communication terminals in mobile telecommunication systems has become very popular. Conventional systems are controlled by at least one mobile services switching center, or MSC, at least one base station, or BS, serving at least one mobile station. The MNC constitutes an interface between the radio-based communication system and the public switching telephone network, or PSTN. The base station acts as a conduit for information between the mobile stations and the MSC. Calls to and from mobile subscribers are switched by the MSC which in addition provides all signalling functions needed to establish the calls.
In order to provide adequate radio coverage of the service area, plural base stations are usually required. The service area is usually divided into so-called macrocells, each normally served by its own base station (in some cases it can share a base station with a number of other cells). Each macrocell can provide communication services via radio link between the macrocell base station and mobile stations (terminals) located in this macrocell, usually in the range of about several kilometers. Assigned to the macrocell are a number of unique communication channels which are usable throughout the macrocell area, i.e. only a single mobile communication terminal operating within the macrocell area may be assigned to any particular macrocell communication channel.
Macrocells are deployed during the initial rollout of a cellular network to provide wide-area coverage. As the cellular network matures, the need for more system capacity arises and one proposal for meeting this need is deployment of so-called microceils as underlay of an existing macrocell. The macrocell area is divided into a number of smaller geographical areas, or microcells, typically of several hundred meters in diameter. Associated with the microcells are a number of microcell communication channels. Certain microceil areas share the same channels, i.e. multiple mobile communication terminals can be assigned to the same communication channel as long as all of the terminals are located in different microcell areas which are geographically separated. Because of geographical separation and assuming low power levels employed with terminals served by microcells, no interference would occur thereby enabling more terminals to be used within a particular macrocell without the need to preempt additional channels. Such a network configuration is often referred to as hierarchical micro-/macro-cellular network.
Though hierarchical micro-/macro-cellular networks allow for much more efficient use of frequency spectrum and hardware resources, there are some problems associated with their operation. After a call in a cell is set up, the quality of the radio link must be monitored to ensure high quality uninterrupted service during the communication session. If the quality of the radio link drops below a predefined level or the terminal moves between cells, the assigned communication channel should be changed or the call should be switched to another base station. Since each microcell covers a relatively small area, moving terminals are likely to traverse multiple microcells during a particular communication session. As a result, multiple microcell communication channels have to be used and multiple transfers or "handoffs" would be required, tying up multiple channels and transfer resources for a single communication session. During peak use periods or in congested urban areas there may be no other target base station receiving a signal of sufficient strength from the terminal or/and no free channel to be used to implement a required handoff. This can lead to an unacceptable call connection deterioration and the call can be lost altogether.
A number of techniques have been used to cope with these problems. Call blockage has been avoided during periods of high demand (when a cell may not have unallocated channels available for handling new calls or call handoffs) by giving microcell-to-microcell handoff requests priority over new call requests originating from a microcell. U.S. Pat. No. 5,301,356 to Bodin (1994) discloses a system and method for ensuring that handoff requests take priority over new call requests to engage voice channels assigned to a specific target cell. According to this technique, a determination is made whether any voice channels of the target cell are available for assignment to call requests. If none is available, handoff requests to the target cell are stored in a queue for a predetermined period of time. New call requests are denied to be served until the queue is empty.
Similarly, U.S. Pat. No. 4,670,899 to Brody (1987) describes a method to avoid call blockage during periods of high demand when a cell may have no free channels available for handling new calls or call handoffs. In this case, the cell utilization is monitored to determine the mode of operation each cell will be directed to. In one of the two predetermined modes of operation voice channels are reserved for incoming handoffs by denying access for mobile transceivers initiating new calls. Such a microcell traffic management technique helps to protect already established calls from being dropped due to a lack of free channels to make an urgently needed handoff, but does not ensure efficient use of the overall communication system capacity.
Another idea is to temporarily use currently available macrocell channels as a backup for microcell communication traffic. The most radical proposal is not to allow microcell-to-microcell handoffs under any circumstances. Any handoff occurs only via the macrocell layer (U.S. Pat. No. 5,278,991 to Ramsdale, 1994). After the terminal has been served by the macrocell for a sufficiently long period, the downward macrocell-to-microcell handoff is performed. Though substantial improvement in the grade of service for the microcell calls can be achieved through implementation of this idea, it can place a great strain on the macrocell layer because handoffs originating from a microcell compete for communication channels with macrocell-to-macrocell handoffs and new calls originating from the macrocell.
A more moderate approach disclosed in the U.S. Pat. No. 5,548,806 to Yamaguchi (1996) and No. 5,396,645 to Huff (1995) is based upon the estimation of the moving terminal speed at a predetermined interval. The terminal is allocated a channel to the microcell base station when moving slowly, and is allocated a channel to the macrocell base station when moving rapidly. Thus, frequent handoff between microcells of rapidly moving terminals can be avoided. Though this approach allows for more flexible interaction between microcell and macrocell layers, it does not take into account the traffic characteristics in both layers of the network. It will not help, for example, in a situation when the number of slowly moving terminals exceeds the microcell traffic capacity. In some cases microcell-to-macrocell handoff can take place, for example, even if free channels are available in the microcell layer.
In summary, known methods for using macrocell channels to carry overflow microcell traffic give an improvement in the grade of service (measured by call blocking) for microcell calls, but at the expense of the grade of service experienced by macrocell calls. The problem of poorer grade of service for macrocel subscribers (when macrocell channels are shared with microcell calls) worsens under heavy macrocell traffic conditions.